The transport of radio frequency (RF) signals over optical fibers has many applications in the cellular and personal communication systems (PCS) industry.
Basically, most voice and data is carried through the public switched telephone network as digital data. For a voice channel, this data is typically 64 kbit/sec. However, this format is incompatible with what a cellular or PCS provider needs to broadcast through the air. In the case of a typical analog cellular voice channel, the 64 kbit/sec channel is first converted to analog, and then to a 30 kHz wide FM signal, and finally this 30 kHz signal is upconverted to near 800 MHz to be broadcast through the air.
For the new generation of digital cellular or PCS even more conversion is required. First, the 64 kbit/sec signal is digitally compressed to a lower bit rate to save spectrum. Thereafter, other signal processing is performed, such as code or time division multiplexing (depending on which digital air interface is used), before the signal is upconverted to its RF frequency. All this conversion requires a substantial amount of electronics, which is located at the cellular base station. Furthermore, presently, in the industry, there is a desire to use smaller cell areas called microcells to provide cost effective coverage of dead spots and to increase capacity. However, the necessity of duplicating all the electronics required for the conversions described above reduces the advantages of this microcell approach.
The concept of transport over optical fibers involves carrying the signal intended for broadcast over the antenna in a form as close as possible to its final one, so that no conversions are required at the microcell site. Such an approach is described in the following publications: W. E. Stevens and T. R. Joseph, "A 1.3 .mu.m Microwave Fiber-Optic Link Using a Directly Modulated Laser Transmitter," IEEE J. Lightwave Tech., Vol. LT-3, No. 2, pp. 308-315 (1985) and D. M. Fye, "Design of Fiber Optic Antenna Remoting Links for Cellular Radio Applications," Proc. 40th IEEE Vehic. Tech. Conf., pp. 622-625, June 1990. In such a case the signals are carried at the RF frequencies which are near 800 MHz for cellular and near 1800 MHz for PCS. These very high frequencies are the reason optical fibers are required. To carry the signals over coaxial cable would require numerous microwave amplifiers which could degrade signal quality.
In the past, systems for RF transport over optical fibers involved the use of a star configuration or architecture, with point-to-point transmission between the base station and each microcell. Such a system involved the use of a separate single pair of fibers to connect a base station with each microcell. If two or more microcells were served from one base station, then separate additional pairs of fibers with associated lasers and photodetectors were required between the base station and each of the additional microcells. This type of arrangement was extremely expensive. Furthermore, in the event of a break in an optical fiber between the base station and the microcell, the RF transport link ceased to function, causing the microcell to fail and operation to end.
It is therefore an object of this invention to provide a reconfigurable ring system for the transport of RF signals to a plurality of remote units such as cell areas or microcells.
It is a further object of this invention to provide a reconfigurable ring system which enables a plurality of microcells to be served by a single pair of optical fibers interconnected between all of the remote units.
It is another object of this invention to provide a reconfigurable ring system for the transport of RF signals which permits all the remote units within the system to remain fully operational even in the event a pair of fibers is cut or damaged.
It is still a further object of this invention to provide a reconfigurable ring system for the transport of RF signals which, even in the event of failure of one of the remote units, permits all the other remote units in the system to remain fully operational.